SUMMARY Alcohol use disorder is estimated to affect 17 million Americans, with great costs to the individual and to society. Although available therapies and counseling can be effective in some drinkers, long-term recovery remains difficult to achieve. Glucagon-like peptide 1 (GLP-1) is a peptide that has both hormone and neurotransmitter functions. It is produced in the digestive tract and regulates blood sugar and food intake, and GLP-1 receptor agonists are used clinically to manage type 2 diabetes. GLP-1 is also produced in the brain, and GLP-1 receptors are expressed in brain regions important in addictions, such as the ventral tegmental area and the nucleus accumbens. Emerging evidence suggests that one or more GLP-1 receptor variants are associated with alcohol use disorder and modulate effects of ethanol in humans, and that GLP- 1 receptor agonists can reduce ethanol intake in rodents. The long-term goal of this research, in a collaboration spanning molecular and behavioral studies in laboratory animals to a clinical trial, is to validate and optimize GLP-1 receptor agonists as a treatment approach for alcohol use disorders. The goal for the present studies is to understand how GLP-1 receptor agonists modulate alcohol use, at a circuit/mechanism level. Aim 1 is to identify the GLP-1 receptor populations necessary and sufficient to mediate decreases in ethanol intake, using microinfusions of the GLP-1 receptor agonist Exendin-4 or the GLP-1 receptor antagonist Exendin(9-39) into the ventral tegmental area, nucleus accumbens, paraventricular region of the hypothalamus, or central region of the amygdala in mice. Systemic administration of Exendin-4, and of a non-brain penetrant Exendin-4 fusion molecule, will be evaluated for comparison. GLP-1 receptor ligands modulate food and fluid intake, making it difficult to separate effects on ethanol seeking specifically from effects on consummatory behaviors generally. Therefore, the present studies will make use of intravenous ethanol self-administration as a non-oral assay of ethanol reinforcement, as well as of a drinking relapse assay. Aim 2 is to determine the effects of Exendin-4 treatment on ethanol-induced neuronal activity, both acute and persistent changes induced by ethanol intake. This aim will use immunohistochemical detection of the immediate early gene c-Fos as a marker of neuronal activity, in brain slices from mice allowed to self- administer ethanol intravenously for at least three weeks, or to drink ethanol daily for at least four weeks. Control groups will self-administer saline and drink only water, respectively. In striatal tissues, c-Fos expression will also be evaluated at the cellular level, distinguishing between direct and indirect pathway medium spiny neurons by use of transgenic mice expressing a fluorescent reporter in specific neuron types. Blood ethanol levels will be determined in all experimental groups.